Exact same problem on my other Wii U. Adding the DLC causes it to hang in a loop on the into sequence. No way to exit, have to pull the plug.
I ended up deleting the DLC, playing to the first save point and then reinstalling the DLC but it still hangs on the initial splash screen.
Decided to delete the game and download the free trial version. That way it will have a legitimate ticket for the game.
Hopefully that will fix it but the DL process is so slow, look like an hour and 10 minutes.
edit: Nope - downloaded the real free one, installed the DLC and it is still hanging at startup. Something must be wrong with the DLC.